1998 Chevrolet Tahoe Newly Bought Discovered That Gaz Consumption Good Making 6

I newly bought a Chevrolet Tahoe and discovered that the gaz consumption is not good at all it is making like 6 mpg I have consulted a mechanical garage and asked them to check my oxygene sensors but they said that these oxygene sensors are cancelled from this Tahoe and they programmed it to function without the oxyg sensors but unfortunately the MPG is still at 6. I need your opinion on what needs to be done and if the oxygene sensors are a must noting that I did a full revision on the sparks oil and filters and the Tahoe is functioning well with the exception of the fuel consumption . Waiting your reply